This invention relates to an electrode assembly for the electrosurgical removal of tissue immersed in an electrically conductive liquid such as normal saline, and primarily to an electrode assembly for housing in an endoscope, for performing, e.g., transurethral prostatectomy (TURP).
In International Patent Applications Nos. WO 97/00647, WO 97/24994, WO 97/24993, WO 97/00646, WO 97/48345 and WO 97/48346 the applicants disclose a number of bipolar electrode assemblies for mounting on the distal end of the elongate tubular instrument shaft. In each case, the electrode assembly is designed for operation whilst immersed in a conductive liquid, typically normal saline, through which current flows from a tissue treatment electrode placed on or adjacent tissue to be treated, to a return electrode which is spaced back from the tissue treatment electrode away from the tissue surface. An electrosurgical generator suitable for supplying power to the disclosed electrode assemblies is described and shown in the applicant's co-pending European Patent Application No. EP 0754437. This generator provides for different modes of operation, a first mode being a tissue desiccation or coagulation mode in which the peak voltage applied between the electrodes is limited to prevent vapour pocket formation at the tissue treatment electrode, and a second mode in which tissue is vaporised to produce a cutting or bulk removal effect at an operation site. During the second mode the power supplied to the electrode assembly causes the formation, from the conductive liquid, of a vapour pocket around the tissue treatment electrode. In this case, the peak voltage applied to the electrode is limited to control the size of the vapour pocket and to prevent electrode destruction. A third mode of operation is a blended mode achieved by switching between the electrical conditions for the first and second modes.
The full subject matter of the above-mentioned co-pending applications is incorporated in this specification by reference.
The electrode assemblies are typically introduced to a body cavity through the working channel of an endoscope inserted through a natural body orifice or through a separate aperture formed to obtain access to the cavity. In either circumstance, the tubular instrument shaft provides the return path for electrosurgical currents, connection to the tissue treatment electrode being made through an insulated conductor passing through the shaft interior. The tubular member also provides for heat transfer away from the electrodes during operations. Thermal dissipation from the electrodes is enhanced by a portion of the shaft being immersed in the conductive liquid.
Endoscopic urological surgery is performed routinely to treat pathologies of the urinary tract using a range of sophisticated instruments introduced through the urethra. Resectoscopes are a specific form of endoscope originally developed for urological surgery. They have since been used in hysteroscopic and gastrointestinal surgery for removal of soft tissues. Resectoscopes differ from many other endoscopes in that they include an integral trigger mechanism to produce a controlled forwards and backwards motion of an instrument attached to the mechanism. This control is particularly useful during removal of large volumes of tissue and as such, they are the instrument of choice for performing TURP, the removal of benign overgrowth of the prostate gland as well as endometrial and fibroid resection during hysteroscopic surgery and resection of polyps and tumours in the rectum during endoscopic gastrointestinal surgery.
Irrigating solutions may be delivered by continuous or intermittent flow through the resectoscope and may be electrolyte or non-electrolyte based. As the traditional technique for performing TURP is monopolar electrosurgery, a non-electrolyte is most commonly used. Conventional instruments, then, generally comprise a range of monopolar electrodes mounted on the resectoscope. A bipolar instrument is known from U.S. Pat. No. 4,116,198 (Roos). This has a single active electrode in the form of a reciprocable resection loop and a return electrode mounted on the distal end of an instrument shaft. Electrical conduction between the electrodes occurs via a conductive liquid immersing both electrodes.
A resectoscope consists of four main components: an inner sheath, an outer sheath, a telescope and light source assembly, and a working element. The working element, whether passive or active, comprises a reciprocating mechanism mounted on a tube. The tube has a telescope connector at its proximal end and a sealing block located part way along its length, to which the inner sheath connects. The sealing block has a hole through it to allow the telescope to be passed from the proximal to the distal end of the working element, within the bore of the inner sheath. The hole is offset so that the telescope is located in the upper quadrate of the inner sheath aperture to make room for the electrode support tube.
A monopolar electrode supported on a wire-form conductor is inserted through the support tube from the distal end through a second hole in the sealing block. The hole is angled so that the electrode exits the sealing block at an increased distance from the telescope, in order that the electrode passes into the insulation block with sufficient insulating material between the electrode and the telescope to provide electrical isolation. These monopolar electrodes are typically of a wire shaft construction to facilitate introduction through the resectoscope with a large working tip of a wire loop or roller ball configuration. A roller electrode is disclosed in U.S. Pat. No. 5,599,349 (D'Amelio).
It can be advantageous to use an electrode supported by wires with conventional endoscopes, the electrode being loaded in the working channel from distal end to proximal end as opposed to conventional loading from proximal end to distal end. The latter loading technique limits the dimension of the working tip of the electrode to the internal diameter of the working channel.
Additionally, wire-form support to an electrode may be useful in circumstances where access and manoeuvrability are restricted by the confines of the body cavity.